The objective of this Program Project is to understand the mechanisms of peripheral tolerance and why tolerance fails in systemic and tissue-specific autoimmune diseases. Common themes in the program are the concept that proximal signals in T cells influence the choice between tolerance and autoimmunity, and the use of genetic disruptions in mice that result in breakdown of immune tolerance. The proposed studies rely on combined functional and biochemical analyses, with a strong emphasis on in vivo model systems. The four component projects are highly complementary in terms of scientific themes, expertise, and tools and technologies. 1. Mechanisms of T cell tolerance vs. autoimmunity in vivo. (P.I.: A. Abbas). The goals of this project are to analyze the roles of two mechanisms of tolerance, anergy and deletion, in tolerance to transgene-encoded systemic and tissue-restricted secreted and membrane forms of "self" antigens. We will define the consequences of T cell encounter with self-antigens, the mechanisms of anergy induction in vivo, and the roles of CTLA-4, Fas, cytokines (IL-2, TGF-beta), and infections in determining the choice between tolerance and autoimmunity. 2. CTLA-4 based tolerance in autoimmune diabetes (P.I.: J. Bluestone). The goals of this project are to examine the role of CTLA-4 in T cell tolerance and suppression in vivo, and the molecular basis of CTLA-4 mediated T cell regulation. In addition, novel membrane-bound single chain Fvs and dimeric complexes will be developed to selectively ligate CTLA-4 in close proximity to TCR engagement, and tested for their ability to induce tolerance and treat autoimmune disease. 3. Regulation of CD45 signaling in tolerance and autoimmunity (P.I. A. Weiss). The goals of this project are to exploit a newly discovered model of autoimmunity caused by a genetic disruption that prevents CD45 dimerization, as a model for defects in signaling receptors that interfere with self-tolerance. The cell lineages responsible for autoimmunity and the role of antigen will be defined, and the molecular basis of CD45 regulation of lymphocyte responses and self-tolerance will be examined. 4. Src tyrosine kinases and autoimmunity (P.I.: A. DeFranco). The goals of this project are to examine the mechanisms by which deletion of two kinases important in lymphocyte responses, Lyn and Fyn, leads to systemic autoimmunity. The cells involved in autoimmunity will be defined, and the molecular mechanisms by which these kinases contribute to lymphocyte regulation and self- tolerance will be examined. The two scientific cores (A. Animals, and B. Flow cytometry) will support all four projects, and develop new mouse strains and analytical techniques for the proposed studies. Thus, the information gained from this Program Project will answer fundamental questions about the choice between self-tolerance and autoimmunity and may provide new strategies for inducing tolerance.